All things were equal – on Equinox Day

One of the biggest misconceptions, even among educated people, is that the reason for the seasons is that Earth moves closer to the sun in summer, and farther away in winter. This is wrong.

The reason for the seasons is tilt.

Nothing else.

Earth’s axis, the line going through the planet from the north pole to the south pole, is always (at least for a human lifetime) pointed to the same place in the sky. But this direction is tilted by about 23.5 degrees when compared to the plane of Earth’s orbit.

With this tilt, half the time the north pole will be pointing slightly toward the sun (the seasons we call spring and summer) and half the time it will be pointing slightly away (fall and winter). The equinoxes – the first day of fall (Saturday), and the first day of spring – are the in-between points.

If you happened to be standing on the equator Saturday, the sun passed directly overhead, or right through your zenith point. It was one of the two days that happens. From our perspective in Durango, the sun is just moving a little bit farther south every day. With less overhead sun, the nights are getting cooler and the leaves are changing colors.

The word equinox literally means “equal night,” so, on the equinox, the daylight and nighttime are 12 hours long. And that is true for everywhere on Earth. The fact that the Earth’s axis is pointing at a right angle to the sun – neither toward it nor away from it – is true for the entire planet, not just for dwellers in one hemisphere or another.

Mars has been in the news recently because of the Curiosity rover’s successful landing. Remarkably, the Mars day, or “sol” is only about 40 minutes longer than an Earth day. This is close enough to an Earth day for scientists working on the Curiosity project to shift their schedules to always be at work during the Mars daylight hours.

Besides the length of day, the other thing about Mars that is remarkably close to Earth’s, is that the axial tilt is about 25.2 degrees. That is within 2 degrees of the Earth tilt. That means Mars has seasons of similar extremes as Earth. The seasons are much longer, because the Martian year is much longer. The Mars year is 687 Earth days, or getting close to two Earth years.

This month

Only at the equinoxes will the sun rise due east and set due west. You might go out at sunset to verify this. If you go out as it is getting dark, you might catch Saturn just before it sets. Mars will set not too long afterward, but because Earth is moving away from Mars right now, it is dimmer than first magnitude.

The Milky Way is still high in the sky. Looking to the south, just to the left of Scorpius, is the teapot asterism that is part of the constellation Sagittarius. The Milky Way is the steam rising from the spout.

A couple of degrees above the lid of the teapot is the globular cluster M-22. Although this cluster can barely be seen with the naked eye, it is a great binocular target. In binoculars it will look like a little cotton ball, but in a 6- or 8-inch telescope you can begin to resolve individual stars in the cluster. The bigger the telescope, the more stars you can see. A telescope like the Hubble can resolve stars down to the core.

hakes_c@fortlewis.edu. Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.